Vitrification Treatment of Asbestos Waste with Incineration Ash of Solid Waste

Transcription

1 High Temp. Mater. Proc., Vol. (11), pp Copyright 11 De Gruyter. DOI.1515/HTMP Vitrification Treatment of Asbestos Waste with Incineration Ash of Solid Waste Eiki Kasai, 1; Hiroshi Goto 2 and Yusuke Mase 1 1 Institute of Multidisciplinary Research for Advanced Materials (MRAM), Tohoku University, Sendai, Japan 2 Institute of Mogami Environmental Chemistry, Shinjo, Japan Abstract. Asbestos fibers have been widely used in various industrial products due to their excellent properties such as thermal stability, chemical resistance and electric insulation. However, it has become to be widely recognized that long-term heavy exposure to airborne asbestos fibers led to serious respiratory diseases such as fibroid lung and malignant mesothelioma. Therefore, its usage is being withdrawn and proper treatment of their wastes is being required. Major asbestos minerals so far used are chrysotile, crocidolite and amosite, and they have relatively high melting temperatures. Further, actual melting temperatures of their wastes are highly depending on the coexisting materials, since most of them have been utilized as mixtures with other construction materials such as mortar. In this study, a simultaneous melting treatment of asbestos wastes and incineration ash of solid wastes was proposed in order to increase flexibility for controlling the fluidity of the formed slag melt during their virification treatment. In addition, a simple and convenient test method was proposed to quickly evaluate the high temperature property of asbestos wastes. By applying this method, a number of the melting experiments were carried out under different conditions using chemical reagents and asbestos wastes, incineration ashes, candidate fluxing materials and slags sampled from an actual melting furnace. Further, in order to control the slag melt property on site, several series of the verification tests were carried out using a practical melting furnace for incineration ash of industrial wastes. It was confirmed that the proposed way was effective to promptly adjust the slag melt fluidity through the evaluation of its composition by the melting test. Keywords. Asbestos waste, vitrification treatment, slag composition, fluidity of melt, fluxing material. Corresponding author: Eiki Kasai, Institute of Multidisciplinary Research for Advanced Materials (MRAM), Tohoku University, Sendai , Japan; kasai@tagen.tohoku.ac.jp. 1 Introduction Asbestos is a mineral group of extremely fine fibrous silicates showing high strength and thermal stability. It has been used as textiles, cement products, brake linings, filters, roofing tiles, flooring etc. in various industries. Chrysotile is a most common asbestos mineral of which ideal chemical composition is Mg 3 Si 2 (OH) 2 and is therefore 3MgO 2SiO 2 after calcination. Its completely-melting temperature, i.e., liquidus temperature, is over 18 ı C(D 73 K). Other asbestos minerals such as crocidolite and amosite also have high melting temperatures. However, since asbestos is usually used as combined materials with other materials, the average composition and the composition segregation are important factors in its melting treatment processes. For example, when the wastes contain a large amount of cement, they show higher CaO content. However, they can be completely melted at lower temperature than 1 ı C, when they are mixed with materials containing proper amount of SiO 2 and Al 2 O 3. In other words, proper slag having high fluidity at low temperature will be formed, if proper fluxing materials are added by appropriate amounts depending on the composition of the wastes to be treated. Nonetheless, it is difficult to constantly measure the composition of the wastes charged to the melting furnace, which significantly fluctuates with time. It is essential to grasp the operational state of the furnace, especially the discharging state of molten slag, and promptly reflect them to the control of the charging materials. At the same time, a large amount addition of fluxing materials should be avoided to control the slag composition, because it will lead to a decrease in the treating rate and an increase in the fuel cost. In this study, a simple and easy-to-use method for the melting test of asbestos wastes was proposed to promptly evaluate the fluidity of their slag melt. Then, several series of fundamental melting tests were made using chemical reagents and actual waste and slag samples. Further, in order to increase the flexibility in the slag fluidity and to alleviate the fluctuation and segregation of the composition of the wastes, a mixed treatment process of the asbestos wastes with incineration ashes was proposed. Then, its validity was verified by using a practical ash-melting furnace for industrial wastes. Received: January 18, 11. Accepted: April 15, 11.

2 354 E. Kasai, H. Goto and Y. Mase 2 Melting Test of Slag 2.1 Measurement of Flow Length at High Temperature Preliminarily grounded incineration ash, asbestos wastes and fluxing materials such as sand and clay were mixed by a ball mill and the obtained mixture was press-shaped into a columnar tablet of mm in diameter. The tablet was placed at the center of the boat-like alumina crucible with 2 mm in length. Then, it was set in a fused silica tube as shown in Figure 1. An infrared furnace was used to rapidly heat and cool the sample. Further, a cylinder made of platinum foil with.5 mm thickness was attached inside surface of the fused silica tube in order to enlarge the soaking area of the furnace. Sample temperature was measured by a thermocouple placed at the center of the alumina boat. Standard heating rates were 3 ı C=min below ı C and 1 ı C=min over to 13 ı C. Then, the sample was held at this temperature for 5 min. After cooling, the length of the molten sample was measured in the long direction of the boat. The difference between the measured value and initial thickness of the sample tablet was defined as flow length of the sample and used as an index of fluidity of the melt. Chemical reagents were used as well as actual wastes and slags in order to obtain the database for the flow property of the melts with a wide range of composition. Sorting of the melted samples was also made considering their colors, surface conditions and crystalline/glassy states. Flow length [mm] Flow length [mm] Estimate viscosity (Pa s) 35 (B) (A) Estimated liquidus temperature ( o C) 2.2 Test Results and Discussions The results for the mixed samples of CaCO 3,SiO 2,Al 2 O 3, MgO and Fe 2 O 3 reagents heated under the standard condition (held at 13 ı Cfor5min)areshowninFigure2. Composition range of these samples is SiO 2 : 4-6, CaO: 15 55, Al 2 O 3 : 5, MgO: 5 and Fe 2 O 3 : 5 mass%. Figure 2(A) and(b) shows the relations between the measured flow lengths and the viscosity at 13 ı C calculated using the equation for the ironmaking blast furnace slag proposed by Iida et al. [1], and the liquidus temperature estimated by the phase diagram of CaO-SiO 2 -Al 2 O 3 -MgO system [2], respectively. A certain correlation can be seen in Figure 2(A), but not for (B). It suggests that the decrease Figure 2. Relationships between flow length of slag and estimated (A) viscosity and (B) liquidus temperature. in the liquidus temperature of the slag does not always leads to its smooth flow and discharge from the melting furnace. Figure 3 shows the iso-flow length lines drawn based on the results measured for the samples of the [asbestos waste]-[incineration ash]-[fluxing material (Clay)] system. It is obtained by a series of the measurements with varying 4 Flux (Clay) 6 Infrared furnace Alumina boat Platinum foil Fused silica tube mm Incineration ash Asbestos waste (mass %) Asbestos waste Figure 1. Schematic diagram of the fluidity evaluation method using an infrared furnace. Figure 3. Estimated iso-flow length lines on incineration ash-asbestos waste-flux (Clay) diagram.

3 Vitrification Treatment of Asbestos Waste with Incineration Ash of Solid Waste 355 (mass%) SiO 2 CaO Al 2 O 3 MgO T-Fe Liquidus Asbestos waste ı C Incineration ash ı C Clay ı C Table 1. Chemical composition and liquidus temperatures of materials used. their mixing ratios. It is found that the region of large flow length over 4 mm appears at the position around [asbestos waste W Incineration ash W Clay] D Œ35 W 55 W in mass%. Chemical composition and estimated liquidus temperatures of these materials are listed in Table 1. The flow lengths measured for the asbestos waste and the incineration ash are less than mm and a little greater than mm, respectively. Further, the clay does not melt at 13 ı C. These results shows that the composition of slag can be directed to region having higher fluidity by the additions of an incineration ash and a small amount of clay as a fluxing material, since the asbestos waste used in this study has high basicity. Further, it suggests that more effective melting treatment operation at lower temperature will be possible through the proper control of the slag composition. 3 Verification Test Using Incineration Ash Melting Furnace 3.1 Procedure It will not be very difficult to control slag composition if the compositions of the asbestos wastes and incineration ashes are preliminarily obtained and their segregations are not significant. For example, their appropriate mixing ratio can be determined by the corresponding phase diagrams such as one for CaO-SiO 2 -Al 2 O 3 (-MgO) system. However, ash composition remarkably changes with the original wastes and composition of the asbestos wastes are also differs depending on the lot. These can be an essential cause of the time fluctuation in the slag composition during operation of the melting furnace. Therefore, this study aims at a rapid control of the slag composition applying an on-site evaluation of the slag fluidity. The following is the procedure of the verification test using a practical ash-melting furnace: i) First, the operational condition of the furnace is evaluated considering the furnace temperature, charging rate of wastes, discharging state of molten slag and so forth. ii) Flow lengths of the slag sample and those mixed with fluxing materials, asbestos wastes etc. are measured. iii) Referring the above results and the morphology observation of the samples after flow length measurements, the optimum mixing ratio of asbestos waste, incineration ash and fluxing materials is determined to direct the slag composition to the appropriate range. Determine new charging ratios of asbestos, ash and flux Measurement of flow length of slag & its mixtures with fluxing materials Not good Judgment of furnace condition (temperatures charging rate) Good Slag production Melt flowing state Good Maintain current operation Bad Flame shape Fuel supply Preheating air Air ratio Figure 4. Flow sheet for leading the slag composition giving higher fluidity in the melting treatment process of asbestos wastes. iv) Furnace operation is made based on the direction proposed above and the operational condition is reevaluated. v) If there is still problem, the procedures from i) to iii) is repeated. The above-mentioned flow is schematically shown in Figure Facility for Verification Test Verification test was made by using a practical melting furnace for incineration ash of solid wastes, which was run by an industrial waste treatment company, Mogami Clean Center, in Yamagata Prefecture, Japan. It conducts the vitrification treatments of own incineration ash formed in the rotary-kiln type incinerator and the consigned wastes including asbestos wastes. The melting furnace has an independent charging slot for asbestos wastes separately from that for incineration ash. They are first mixed and put into the furnace by a pusher and through a screw-feeder. An apparatus to feed fluxing materials was additionally installed for the verification test. The furnace, 7 mm in inside diameter, is so called a surface-melting type and provides four heavy oil burners (Figure 5). Its inside wall is made of alumina refractory covered by a water-cooling jacket. Molten slag flows out from a tap located at the side bottom of the furnace and is water-granulated. The outlet gas is sent to

4 356 E. Kasai, H. Goto and Y. Mase A C [Cross-section A-B-D] Screw conveyer for Incineration ash B φ 7 Burner D Air Preheated air Heavy [Cross-section C-B-D] oil Molten slag Water granulator Conveyer for granulated slag Slag container Water-cooled wall Refractory Outlet gas to heat exchanger Slag outlet Figure 5. Outline of the melting furnace of incineration ashes used for verification test (Mogami Clean Center). a bag-filter through a heat-exchanger and a rapid cooling tower. ( o C) (mm) (L/h) (kg / h) Outlet gas temperature Flow length of sampled slag 1 Rate of fuel (heavy oil) supply Production rate of slag 1 Charging rate of asbestos waste 9: : 11: 12: 13: 14: 15: 16: 17: Charging period of asbestos waste and slaked lime 12 kg/h Clay 4.9 kg/h Clay 9.7 kg/h Figure 6. Changes in the main process parameters obtained in the 2nd day of the 2nd series of the operation experiment. 3.3 Measured Items During the Verification Test Fuel supplying rate, temperatures of preheated air, outlet gas and discharged slag, slag forming rate and outlet gas composition (CO, CO 2,O 2, SOx, NOx and HCl) were measured during the verification test. Further, charged incineration ash, discharged slag and fly ash in the bag filter were sampled at a regular time interval. These were substituted to the analyses of chemical composition, residual asbestos fibers and the dissolution and content tests designated by the Japanese Soil Pollution Control Act. In addition, measurements of residual asbestos in outlet gas and suspended asbestos in the working environment were made throughout the test. 3.4 Results of the Verification Tests Three series of the verification tests were conducted. Each series was performed for about one week including the warming-up operation of the melting furnace. The first series of test was devoted to confirm the charging system function of the fluxing materials and the ways of various measurements of the process parameters and sampling of the charged wastes, formed slag and secondary dust. The results of the second and third series are summarized as follows: Incineration ash used in the second series of the verification test had hardly-fusible composition (SiO 2 : 52., CaO: 8.44, Al 2 O 3 : 15.1, MgO: 1.41, T-Fe: 3.63, Na 2 O:.34, K 2 O: 2.86, S:.26 in mass%) and measured flow length showed that its fluidity at 13 ı C was very low. Its reason was estimated to be attributed to relatively high Al 2 O 3 and SiO 2 contents of the incineration ash, considering the preliminary results of the melting experiment obtained for different slag samples. In order to manage it, slag fluidity was first attempted to increase by the addition of slacked lime as a fluxing material. This effect was checked during the preliminary operation with charging the incineration ash only, and then the verification test was started by charging the asbestos wastes. Figure 6 shows the changes in the major process parameters obtained in the second series of the test. The slacked lime addition at 12 kg=h started on 8:45. Then, the charge of the asbestos waste was started on 9:. The target value of the charging amount of asbestos waste was % of the total charge. However, its level varied within the range between 15 and %, although it reached once to the target value. It was due to the limitation on the capacity of the charging facilities of the asbestos wastes, which are usually double packed in thick PVC bags with relatively small density. Dusty and fibrous asbestos wastes, so called flyable asbestos wastes, such as sprayed materials on the walls and ceilings of the buildings, tend to contain higher amount of CaO and MgO. When charging it to the melting furnace, the slag composition will therefore shift to higher basicity. The addition of SiO 2 and Al 2 O 3 components is often necessary to compensate it. Hence, clay was charged as a fluxing material at 4.9 kg=h from 13:45, and its charging rate was changed to 9.7 kg=h from 15:, which corresponded to about 3 and 6% of the total charge, respectively. This leads to a remarkable increase in the flow length of the slag and the slag production rate of the melting furnace also increased as shown in Figure 6. Confirming a steady furnace condition, the heavy oil supply was able to decrease. On the other hand, the effect of the clay addition did not give a further effect to the slag flow length. This was because SiO 2 and Al 2 O 3 contents of the slag were higher than those

5 Vitrification Treatment of Asbestos Waste with Incineration Ash of Solid Waste 357 ( o C) (mm) (L/h) Outlet gas temperature Flow length of sampled slag Rate of fuel (heavy oil) supply 1 Production rate of slag 4 Charging rate of asbestos waste 8: 9: : 11: 12: 13: 14: 15: 16: 17: Charging period of asbestos waste Slacked lime 6. kg/h) Slacked lime 12. kg/h) 18. kg/h) Figure 7. Changes in the main process parameters obtained in the 3rd day of the 3rd series of the operation experiment. expected, since the amount of supplied asbestos wastes did not reach to the target value. Figure 7 shows the changes in the major process parameters obtained in the third series of the verification test. Slag fluidity and production rate were low until 9:. Asbestos wastes and slacked lime started to charge on 9:, since the flow length measurement of the slag suggested that it would lead to an improvement of fluidity. After a while, the slag production rate increased and stable operation was kept until 12:. Therefore, the heavy oil supply could be lowered a little at :. Since the flow length measurements of the slag sampled at 11: suggests that an increase in the slacked lime addition would lead to further improvement of slag fluidity, the amount of its addition was increased to 12 kg=h. Then, the heavy oil supply could be further decreased with confirming the stable operation of the furnace. Almost all asbestos wastes prepared for the test had charged to the furnace at about 15:. In order to compensate for the lack of high basicity components due to the decrease in the charging amount of the asbestos wastes, the amount of slacked lime addition was further increased to 18 kg=h. Then, the slag production rate was kept at a constant level. This made the heavy oil supply possible to decrease to about 1 L=h, which is the minimum level in the whole test series. It appears that the slag production rate increased when the charging amount of asbestos waste decreased (after 14:) as seen in Figure 7. The reason could be attributed again to the less density of the asbestos wastes. In addition, flyable asbestos wastes usually contain a significant amount of water, which was splayed when the asbestos was torn off from the wall to prevent their dispersion and therefore certain heat is necessary for its drying before melting. It may be possible to solve it by preliminary drying the asbestos wastes, e.g., by using recovery heat from the outlet gas of the furnace. It is noteworthy that all the slag samples obtained during the verification tests were passed the heavy metal dissolution and the content evaluation tests for utilization as paving designated by the Ministry of Environment, Japan. Further, the measurements for the concentration of the suspended asbestos fibers in the working environment and in the waste gas of the furnace showed that every evaluated item met respective standard. 4 Conclusion A mixed virification treatment process of the asbestos wastes with incineration ashes was examined, in order to increase the flexibility in the fluidity control of the formed slag and to alleviate the fluctuation and segregation of the composition of the wastes. A simple and convenient test method was proposed to quickly evaluate the fluidity of the molten slag. By applying this method, the way to control the slag composition toward its appropriate region giving higher fluidity was proposed. Then, its validity was verified by the test operation using a practical ash-melting furnace for the incineration ash. The results showed that the actions suggested by the evaluation test of the slag fluidity were effective for improving fluidity and production rate of the slag, and reducing fuel usage. It can treat asbestos wastes safely and reliably without residual asbestos in waste gas, slag, fly ash of the furnace and working environment around the furnace during the operation. Further, it was confirmed that the formed slag was possible to recycle-use, such as for load paving and construction materials. Acknowledgments This study has been carried out as a project of the Grant-in- Aid for Scientific Research, Ministry of the Environment, Japan (K186, K1946 and K59) and their financial supports was greatly appreciated. References [1] T. Iida, M. Ueda, K. Nakashima, Y. Kita and K. Mori, Viscosity of Glass & Slag, AGUNE Technology Center, (3), p [2] G. Cavalier and M. Sandreo-Dendon, Quaternary slags CaO- MgO-Al2O3-SiO2: liquidus surfaces and crystallisation paths for constant magnesia concentrations, Rev. Metall., 57 (196),